1. Field of the Invention
The present invention relates to an LC (liquid crystal) aligning substrate used for an LCD (liquid crystal display), and a method of forming the LC aligning substrate.
2. Description of the Prior Art
LCDs are widely used for notebook computers. The LCDs employ an LC aligning substrate to control the orientation of liquid crystals. Generally, the LC aligning substrate is formed by forming a transparent electrode such as an ITO (indium tin oxide) thin film on a glass substrate, depositing an LC aligning resin film such as a high-polymer polyimide film on the transparent electrode, and rubbing the surface of the LC aligning resin film in one direction with a rubbing cloth made of cotton or rayon to orient the vectors of surface molecules of the film in the direction.
FIG. 9A is a vertical section showing an apparatus for rubbing the surface of such an LC aligning substrate, and FIG. 9B is a plan view showing the same. The LC aligning substrate 3 is fixed to a table 1. A roller 5 wound with a rubbing cloth is rotated at high speed in a direction A. The roller 5 is made contact with the surface of the substrate 3 and is moved in parallel with the substrate 3 in a direction B so that the surface molecules of LC aligning resin on the substrate 3 are oriented in a given direction. Although this example fixes the substrate 3 to the table 1 and moves the roller 5 in parallel with the substrate 3, it is possible to fix the roller 5 and move the substrate 3.
This prior art is suitable for mass production and is low cost, and therefore, most of LC aligning substrates are formed according to this prior art.
Although the rubbing process of the prior art is simple, it is very difficult for the prior art to optimize rubbing conditions. This is because the LC aligning performance of an LC aligning substrate greatly varies depending on LC aligning resin deposited on the substrate, the material of the rubbing cloth, the revolution and moving speeds of the roller, contact pressure between the roller and the substrate, etc. It is very difficult, therefore, to optimize these rubbing conditions.
In addition, the prior art causes a defect called rubbing stripes shown in FIG. 10A. The rubbing stripes 7 appear at regular intervals on a screen when an LCD 15 is turned on. The stripes 7 are thought to be caused by defects on the roller, in particular, on the rubbing cloth because revolutions of the roller match with the intervals of the stripes 7. The defects on the rubbing cloth include a seam of the rubbing cloth and irregularities caused by double-side adhesive tapes attached between the roller and the rubbing cloth. According to our studies, the defects on the rubbing cloth locally and sharply change the orientation of surface molecules of LC aligning resin on the substrate, to affect the orientation of liquid crystals, thereby creating the stripes 7. FIG. 10B shows a seam 13 of a rubbing cloth 9 wound around a roller 5. The seam 13 may produce the stripes 7 of FIG. 10A on an LCD screen.